1. Field of the Invention
The present invention relates to an image pickup apparatus having a standard-resolution photographing mode and a high-resolution photographing mode.
2. Description of the Related Art
The recent development of video apparatuses, such as electronic still cameras or video cameras, is remarkable, and it is desired that each individual video apparatus have a much larger variety of functions against the background of multimedia concepts.
It has heretofore been known that, in a camera arranged to photograph a subject by utilizing an image pickup element, since pixels are discretely arranged, an aliasing distortion or a color moire occurs in the case of a subject having a high spatial frequency in theory. To prevent the occurrence of the aliasing distortion or the color moire, such a camera has a crystal low-pass filter (hereinafter referred to as the "crystal LPF") disposed in an photographing optical path. The crystal LPF serves to prevent the occurrence of the aliasing distortion and a false color by defocusing a subject image by means of its double refraction and suppressing the high-frequency component contained in the subject image to a frequency below the Nyquist frequency determined by the pixel arrangement of the image pickup element.
If the aforesaid camera is utilized as a peripheral for a multimedia system it will be necessary to perform not only color photography but also high-resolution photography such as photography of a document or the like. It is desired, therefore, to increase the number of functions of the camera for the purpose of realizing the high-resolution photography. For example if an object basically represented in black and white, such as a document, is to be photographed, it will be sufficient to use black-and-white photography which involves no false color. In the case of the black-and-white photography, there may be a case where a higher resolution is preferable to the suppression of a distortion. To cope with this case, it has been proposed to provide a camera having a high-resolution photographing mode in addition to a standard-resolution photographing mode for normal photography. The high-resolution photographing mode is a method which is intended to realize high-resolution photography by removing the crystal LPF from the photographing optical path or by changing the crystal LPF for a crystal LPF capable of transmitting a spatial frequency higher than the aforesaid crystal LPF.
However, even in the case of the high-resolution photographing mode, it is impossible to completely remove the aliasing distortion. For this reason, if photography is to be performed in the high-resolution photographing mode it is necessary to set an aperture-diameter optimum for the pixel pitch of an image pickup element used, thereby cutting high-frequency components by means of a reduced aperture diameter. For example, if use is made of an iris of the type which is commonly used in ordinary video lenses or the like, i.e., an iris made up of two blades each having an ND filter bonded thereto for adjusting the amount of lights it is impossible to mare the shape of an aperture opening completely circular. Under the influence of a diffraction due to the incompletely circular shape of the aperture opening, it will be impossible to set the aforesaid optimum aperture diameter. In the case of a fully open aperture diameter which is used during photography in the standard-resolution photographing mode, an inclination of the image pickup element with respect to an optical axis appears as a partial, peripheral defocus. For this reason, it is necessary to specially limit the size of the fully open aperture diameter.
As is apparent from the above description, in the case of the black-and-white high-resolution photographing mode, merely by removing the crystal LPF from the optical path, it is impossible to solve the above-described various problems due to the iris and it is also impossible to achieve a high-resolution photographing mode which is satisfactory in terms of performanes.
In the field of video signal processing for the above-described image pickup apparatus, such as an electronic camera or a video camera, digital video signal processing has recently been becoming popular so that signal processing capable of coping with an image signal of higher-image quality can be realized or so that the exchange of data between the image pickup apparatus and other apparatus can be facilitated.
Conventionally since the digital video signal processing is all performed in an IC digital process circuit it has been impossible to achieve efficient processing. in additions since alteration or the like of the characteristics of the digital process circuit is impossible it is necessary to design a dedicated digital process circuit for each system circuit, so that it is impossible to realize a system having versatility.
For the above-described reasons, it has been impossible to perform the digital signal processing of the video signal smoothly rapidly and efficiently, and it has also been impossible to externally alter various kinds of processing such as signal processing, adjustment or correction in addition, it is impossible to provide a system having good versatility, and it is difficult to realize a wide variety of signal processing corresponding to multimedia techniques.